Over the past 150 years, as greenhouse gas emissions from fossil fuel combustion and other human activities have steadily increased, Earth’s forests, oceans, and soil have helped stabilize the situation by sequestering and storing massive amounts of carbon.
Carbon that is sequestered by oceans and coastal wetlands, known as blue carbon, is especially important, as more than 70% of Earth’s surface is covered by water. Coastal wetlands, from mangrove forests to salt marshes to bright green beds of seagrass, are particularly effective at sequestering carbon. Although they occupy less area than woodlands, coastal wetlands take in carbon emissions and convert them into plant biomass more quickly, are better at trapping organic carbon from within their own ecosystem and other sources, and delay the decay of organic material (which releases carbon) for longer periods. Many coastal habitats are able to store carbon for hundreds, even thousands, of years.
These waterlogged carbon sinks are exceedingly effective but naturally limited; at some point, the ever-increasing amount of carbon in the atmosphere will exceed the amount these sinks can take in. As such, environmental scientists are concerned about how coastal regions will respond to future changes in climate, sea level, geology, and human pressures.
To find out, Kelleway et al. studied coastal wetlands in southeastern Australia that have similar types of vegetation.
The researchers collected samples of mud, water, and other organic matter—cylinders of soft earth, about 5 centimeters in diameter, called cores—at one marine (saline ocean water) site and one fluvial (upstream, brackish water) site at each of two locations: Wapengo Lagoon and Port Stephens.
They then measured the amount of carbon stored at each site and calculated the rate at which it had accumulated by comparing current conditions with detailed environmental histories of each site.
The researchers linked high levels of stored carbon at both of the Port Stephens sites to mangrove tree roots deep below the soil surface. Neither of the Wapengo Lagoon sites contained mangrove roots, but the researchers did detect well-preserved charcoal formed from the burning of plant biomass during wildfires thousands of years ago, representing large amounts of stored carbon.
This study demonstrates the importance of considering both contemporary and historical environmental factors to better understand the ability of coastal wetlands to sequester carbon in our ever-changing global environment. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG003775, 2017)
—Sarah Witman, Freelance Writer